A particle (hadron) composed of a quark and an antiquark of opposite colors. Often a given quark-antiquark pair does not occur on its own, but instead is mixed with others so that the quarks have a superposition of flavors (as always, flavors more similar in mass mix more). The lowest energy mesons are the pseudoscalars (spin 0), where the quark and antiquark have opposite, and then the vectors (spin 1), where the quark and antiquark have parallel spin. Both come in higher energy versions where the spin is augmented by orbital angular momentum. Most of a meson's mass comes from binding energy, rather than the mass of the components. All mesons are unstable.
The first few pseudoscalar mesons: